The present invention relates to a polymer-modified bitumen compound for a bituminous covering layer of a roof sheet. The invention further relates to use of polyethylene as a modifying additive in an SBS-modified bitumen compound. The invention also relates to a method of producing a bituminous covering layer of a roof sheet from a polymer-modified bitumen compound. The invention further relates to a roof sheet having a bituminous covering layer and a bitumen reinforcement and to a method of producing and recycling such a roof sheet.
Such roof sheets are known from the prior art and are used in particular to cover and seal flat roofs. Depending on building requirements, the bituminous covering layer is provided, for example, as a conventional bitumen layer (oxidized bitumen layer) or as a polymer-modified bitumen layer, and the bitumen reinforcement as a polyester fleece, fiberglass mat, fiberglass fabric or felt core.
After reaching the end of their service life, the roof sheets are usually processed for energy recovery, i.e., they are burned as a substitute fuel in coal-fired power stations. This kind of recycling causes considerable CO2 emissions and does not constitute full (material) recycling of the roof sheets in the real sense.
Varieties of distillation bitumen and oxidized bitumen are basically and primarily used nowadays to provide a bituminous covering layer in such roof sheets. Bitumen as such is obtained by distilling crude oil. It consists of a mixture of different hydrocarbons and on account of its properties is one of the most-used sealants in the construction industry. Distillation bitumen, or refined bitumen, usually refers to residual oil that remains after fractional distillation at reduced pressure and a temperature of approximately 350° C. These soft to medium-hard bitumen varieties are primarily used in road construction. Distillation bitumen may also refer to a “high-vacuum bitumen”, which is obtained by further processing or further distillation of distillation bitumen under a vacuum. High-vacuum bitumens, i.e., a special form of distillation bitumen, are mostly used for mastic asphalt, floor screeds and rubber goods, due to their hardness, and can also be used as distillation bitumens for a bituminous covering layer of a roof sheet. In contrast, oxidized or blown bitumens are obtained by blowing air into distillation bitumen air at a temperature of approximately 250° C. Depending on the specific process used, it is possible to give the bitumen improved properties in respect of resistance to heat or cold. This type of bitumen is mostly used for roof sheets, sealing sheets and adhesives. However, it has been found that oxidized bitumens have certain disadvantages with regard to hot processing and/or recyclability, which may be detrimental to their use. Examples of such disadvantages include health hazards or the like, so it may be advantageous in certain cases to use distillation bitumen instead of oxidized bitumen.
Polymer-modified bitumen compounds, so called, are obtained by chemical mixing of distillation bitumen and polymers, and are proven in practice. Their thermal and expansion properties can be varied very well. Polymer-modified bitumens are used for high-quality roof and sealing sheets and for traffic areas that are subjected to heavy loads and stress, such as airports. Parameters which are routinely used to classify different grades of bitumen include depth of penetration and softening point. Breaking point and ductility are basically suitable as well. Other values of potential interest for processing or industrial production include flash point, paraffin content and ash content.
The penetration value is usually specified on the basis of needle penetration pursuant to OENORM EN 426; a particular penetration depth is obtained that specifies the bitumen hardness in 1/10 mm. Since bitumen does not have a melting point, it is necessary to define a softening point by subjecting it to continuous heating. The ring-and-ball method determines a certain temperature at which a specific degree of plastic deformation is reached. This temperature is referred to as the softening point according to OENORM EN 427. A breaking point may be determined in accordance with OENORM EN 12593, for example. Ductility is determined by stretching a test object in an apparatus. The length in centimeters that the test item can be stretched before it breaks is referred to as its stretchability or ductility.
Whereas oxidized bitumen is relatively advantageous in certain respects compared to distillation bitumen, distillation bitumens may nevertheless have a substantial advantage over oxidized bitumens due to their better compatibility in production, use, processing and in their long-term use as roof sheets. This may relate to behavior under heating, for example, which may result in outgassing or the like. It would also be desirable, in the case of distillation bitumens and in particular in the case of polymer-modified bitumen compounds, to provide characteristics that are similar or equivalent to those of oxidized bitumens.